1. Field of the Invention
The present invention relates to a nonvolatile memory device, and more specifically, to a programmable nonvolatile logic switch memory (register) using a resistive memory device.
2. Description of the Prior Art
Generally, nonvolatile memory such as magnetic memory and phase change memory (PCM) has a data processing speed similar to that of volatile RAM (Random Access Memory). When power is off, data of the nonvolatile memory are preserved.
FIG. 1 is a circuit diagram illustrating a conventional volatile logic switch device. Since conventional logic switch and register devices are volatile, data stored in such registers are not preserved when power is off.
A volatile logic switch SW1 connects a node B with a node C in response to a control signal applied to a gate input terminal A. The gate input terminal A of the volatile logic switch SW1 has no memory device for storing previous and current data. As a result, data stored in the memory device are not preserved when power is off.
FIG. 2 is a circuit diagram illustrating a conventional flip-flop volatile memory device which is a SRAM (Static Random Access Memory).
The flip-flop volatile memory device comprises a flip-flop unit including PMOS transistors P1 and P2, and NMOS transistors N1 and N2. The flip-flop volatile memory device further comprises NMOS transistors N3 and N4 for storing data applied from bitlines BIT and /BIT in the flip-flop unit depending on an enable state of a wordline WL.
The conventional flip-flop volatile memory device can store data in both terminals of the flip-flop unit with a static state when power is on. However, the data stored in the terminals both of the flip-flop unit are destroyed when power is off.
Nonvolatile memory devices have been developed to overcome the above problem of the conventional volatile memory device. FIGS. 3a to 3d are diagrams illustrating a conventional phase change memory (PCM) device.
The PCM device 4 comprises a phase change layer (PCL) 2 of phase change material for receiving voltage and current between a top electrode 1 and a bottom electrode 3. The voltage and current induce high temperature in the PCL 2, thereby changing electric conductivity of the PCL 2.
As shown in FIG. 3c, if low current of less than a threshold value flows in the PCM device 4, the PCL 2 has a proper temperature to be crystallized. Thus, the PCL 2 comes to have high resistance.
Referring to FIG. 3d, if high current of more than a threshold value flows in the PCM device 4, the PCL 2 has a temperature over a melting point of the phase change material. Thus, the PCL 2 becomes uncrystallized to have low resistance.
The PCM device 4 can store nonvolatile data corresponding to the two resistances.